1. Field of the Invention
The present invention relates to an optical recording medium, and more particularly, to an optical recording medium with a wobbled track to which a wobble signal containing user data is recorded, and a data recording method and apparatus therefore.
2. Description of the Related Art
An optical recording medium includes a header area to which header information is recorded and a user data area to which user data is recorded. In the case of a 2.65 GB or 4.7 GB DVD-RAM, each sector contains 128 bytes of header information. The header information is recorded in a form of pre-pits during the manufacturing of a disc substrate. According to DVD-RAM specifications, the header area having pre-pits formed during the manufacturing of a disc substrate includes a variable frequency oscillator (VFO) region for phase locked loop (PLL), a physical identification data (PID) region to which sector identification information (ID) is recorded, and an ID error detection (IED) region for storing error detection information. The header area in which pre-pits are formed is disposed at a predetermined portion of a sector. A pickup device provided in a recording/reproducing apparatus can easily find and move to a desirable location based on information recorded on the header area. The pickup device can recognize a sector number, sector type and a land or groove track and perform servo control based on the information recorded in the header area.
As the use of multi media rapidly spreads, a variety of methods for recording more information to an optical recording medium, such as a digital versatile disc (DVD), have been proposed. For example, there are methods of broadening a user data area to which user data can be recorded, including reducing the wavelength of a recording or reproducing laser and decreasing a track pitch.
FIG. 1 is a schematic diagram of a conventional optical disc. Referring to FIG. 1, land and groove tracks corresponding to a user data area to which user data is recorded are formed on the optical disc. Header areas 3 to which header information is recorded in a form of pre-pits are also disposed on the optical disc.
FIGS. 2A through 2D shows examples of a header area of a conventional optical disc. Referring to FIG. 2A, pre-pits for recording header information are formed in the middle the land and groove tracks. In other words, pre-pits are assigned to each track. In this structure, a track and pre-pits are formed on the same circumference so that a wobble signal and the header information can be recorded at the same time. However, if the density of the tracks is increased to improve the recording density, cross-talk may occur during the reproduction of the header information.
Referring to FIG. 2B, pre-pits are formed on a border between a land track and a groove track. In this structure, even if the density of the tracks is increased, cross-talk does not easily occur compared to the structure of FIG. 2A. In addition, a wider area for the pre-pits is provided, allowing the option of increasing the width of the pits. In other words, the structure of FIG. 2B is more preferable than that of FIG. 2A in terms of signal reproduction. However, since the pre-pits are formed on the border between the land track and the groove track, this structure is vulnerable to the tracking offset of a pickup device during recording or reproducing.
Referring to FIG. 2C, a group of pre-pits is formed on the middle of each land or groove track such that the group of pre-pits in one track is not adjacent to another group of pre-pits in an adjacent track. Accordingly, even if the density of the tracks is increased, a probability of cross-talk between the adjacent tracks is very low. However, since the pre-pits are formed on the middle of each track, this structure is insensitive to a tracking error. Therefore, it is difficult to perform smart servo control with the structure shown in FIG. 2C.
A structure shown in FIG. 2D is used in present DVD-RAMs. A group of pre-pits is formed on the border between a land track and a groove track such that the group of pre-pits in one track is not adjacent to another group of pre-pits in an adjacent track. Accordingly, cross-talk can be reduced and smart servo control can be achieved. However, it is difficult to position the pre-pits when the group of pre-pits in one track is not adjacent to another group of pre-pits in an adjacent track during the manufacture of a disc substrate. Accordingly, the signal characteristics of parts constituting a header area may not be in accord with each other.
In a conventional DVD-RAM, the proportion of a header area to a disc area is 0-5% per sector. To increase a user data area by minimizing such an overhead, a dual layer structure having two data recordable sides is used. However, in such a dual layer structure, recording power is influenced by the physical geometry of a lower layer when data is recorded to an upper layer.
To derive an improvement, effects of the physical geometry of a header area on a recording power in a dual layer structure were studied. More specifically, the quantity of light reflected from the structure such as a pit area and a groove area were calculated from simulations and compared to the measured values.
As shown in FIGS. 3A through 3D, the quantity of light reflected from a mirror substrate, a pit area, a groove area, and a groove area with marks was calculated during the simulations. A curvature of 30 μm was applied to a lens to account for the effect of a space layer between a lower layer and an upper layer in a dual layer structure. In addition, the number of tracks of the lower layer captured by laser beams passing through the lens was considered during the calculation.
To measure the quantity of reflected light, conditions as shown in FIGS. 4A through 4C were set. Here, “L0” denotes a lower layer, and “L1” denotes an upper layer. A reflective film l is formed below the upper layer L1. Laser beams are focused on a mirror area in FIG. 4A, on a pit area in FIG. 4B and on a groove area (with no marks) in FIG. 4C.
FIG. 5 is a graph showing the results obtained from FIGS. 3A through 4C. In calculating the quantity of light reflected, a track pitch was doubled to cover the structure as shown in FIG. 2A. For the groove area with marks, only the difference in reflectivity between a marked portion and a land or groove portion was considered. A difference in the transmittance of the marked portion due to a difference in absorptance between an amorphous state and a crystalline state was not considered.
Tables 1 and 2 show input parameters and their values for the experiments.
                                TABLE IParametersValuesWavelength (nm)400Numerical aperture (NA) of an objective0.65lensMinimum mark length (μm)0.275ModulationEFM+Track pitch (TP) (μm)0.30, 0.34, 0.38Reflectivity (%)Rc = 28, Ra = 10
                                      TABLE 2ItemsFactorsExamplesDualTransmittance of L060%layerGeometry of L0Groove, pit, . . .High NANumber of tracks ranged over by aNA 0.65:85 tracksbeamNA 0.85:160 tracksAngle of an incident beam/Reduction ofNA 0.65:40.5°transmittanceNA 0.85:58.2°
According to the simulations, transmittance least decreased at the mirror substrate and decreased more at the pit area. Further decrease in transmittance was observed at the groove area. Depending on the track pitch, the transmittance decreased by 4-7.5% at the pit area while the transmittance decreased by 7.5-28.5% at the groove area.
In the measurements, the quantity of light reflected from the pit area was measured at a track pitch of 0.37 μm and decreased by 0-4%. In the case of the groove area, decrease in a measured value was less than the decrease calculated in the simulations. It is inferred that this phenomenon occurred because while a wall angle of 90° was assumed in the simulations, practical wall angle was 0-60°, so that the measured quantity of the light was 3% larger than the calculated value from the simulations.
As a track pitch decreases, the quantity of transmitted light decreases at the pit and groove areas. A measured value obtained at a track pitch of 0.34 μm (practically, 0.34 μm×2) was 0-4% smaller than a reference value obtained at a track pitch of 0.38 μm (practically, 0.38 μm×2). However, when a header area has a structure as shown in FIG. 2D, it is inferred that the quantity of transmitted light decreases less. A decrease in the quantity of transmitted light was 9.5% at a track pitch of 0.34 μm and 22% at a track pitch of 0.30 μm at the groove area. When a track pitch was 0.34 μm, a measured value was 7.5% smaller than a calculated value.
As a result, it can be inferred that the upper layer L1 needs at least 20% larger recording power than the lower layer L0 at the groove area when a track pitch is 0.30 μm and the NA of an objective lens is 0.85. In other words, a header area with pre-pits is not suitable for high density recording and influences recording power when data is recorded to the upper layer of a dual layer structure.